I have a Vista laptop I use for most of my work, from writing to development. However, i have a Linux box that roughly matches the deployment environment of my main web host, and I can use it graphically via a VNC client from my laptop for the best of both worlds.
For PHP-GTK especially, I find that there is really no alternative to using Linux. It's also great for utilities such as benchmarking tools, caching mechanisms and performance analysers such as
valgrind.
I've also got an XP box that I use for testing new apps, and for Zend Studio (the slowest IDE ever; until I get 3GB of RAM in this thing I'm not letting it anywhere near my laptop).